Articles which have conventionally been formed by plastically deforming a plate-shaped workpiece by bending include articles which have an undercut portion while maintaining a plate-like shape. For example, a lockup damper apparatus of a torque converter disclosed in below-listed Patent Document 1 uses a rotary member called a “piston”. The rotary member is a circular plate which is formed from steel plate and which has a peripheral portion which is raised or bent upward. A distal end portion of the raised portion slants inward to thereby form an undercut portion.